About the only complimentary thing that could be said about the idiotic defacement of the 7 July memorial, on the eve of the ninth anniversary of the bombings, is that the vandals had a sense of architectural context. Architects Carmody Groarke used a minimalist language that is increasingly familiar. Clusters of thin steel stelae, one for each victim, are placed in four corners for the London locations of the attacks. The spray-painted stencils ran vertically along each of them – it looked as if they were made to measure. On them were messages of such Four Lions-esque stupidity – "J7 Truth", "Four Innocent Muslims" – that it's a surprise "Wake Up Sheeple" doesn't appear alongside, but in the process, a mute memorial suddenly had a domineering voice screaming all over it.

Not all memorials today are abstract – viz the clumsy figures of everyone from Lloyd George to Bobby Charlton built in the last couple of decades – but many are. It is deployed when an event is still raw, still able to upset people, potentially interpreted in different ways. It ultimately derives from the Imperial War Graves Commission in the aftermath of the first world war, and in particular, the work of the architect Edwin Lutyens. As outlined in Gavin Stamp's recent book on Lutyens' Memorial to the Missing of the Somme, in Thiepval, northern France, the War Graves Commission, for all its imperialism, had tried to move away from a tradition favouring victory and vainglory in favour of acknowledgement, however tacit, of the pointlessness of suffering. So in commemorating the unfound bodies of the Somme, Lutyens designed an arch of loss, not of triumph – what triumph was there to celebrate?

Similarly, for the Cenotaph in Whitehall, Lutyens managed, impressively, to resist widespread calls for Christian imagery – what about the Jewish, Hindu or Muslim dead, he pointed out – and for any obvious representational sculpture – the lamenting angels and the like previously used in the genre. Instead he designed a delicate yet imposing Portland stone plinth, saying nothing save "Our Glorious Dead" – the second word was as near as he got to triumphalism, topped by a stone wreath. It appeared to argue that something horrible and unprecedented had happened which couldn't be dealt with in the old way.

Stamp would probably argue that the difference between Lutyens' memorial and Carmody Groarke's is that the former still has some affinity with a familiar language, a continuity with the classical architecture of death – the complex tapering proportions, the heavy stone, the wreath – which becomes banal when reduced to simple steel girders, which suggest another, industrial language. But then the other option is, if anything, worse: the kitsch of the neoclassical Bomber Command memorial at the other end of Hyde Park Corner, whose only possible virtue is that it "looks like a memorial", which for most classical revivalists is enough.

In both cases, controversial events – very controversial indeed, in the case of the bombers of Dresden – are made mute, and in this case it is not because of the need to signify loss, but from the need not to say anything in particular. The 7/7 memorial is better than that – like Lutyens' memorials, it avoids sentimentality, condescension, pomposity or moralism, but it too "looks like a memorial" and is essentially familiar. As at Peter Eisenman's Memorial to the Murdered Jews of Europe in Berlin, the freedom of interpretation coincides with a freedom of space, where shapes demarcate an unprogrammed emptiness. Visit the Holocaust memorial to see it used by children as a particularly fiendish maze.

One first world war memorial, not far from the 7 July sculptures, suggests what a "speaking" memorial could be: the Royal Artillery memorial, designed by CF Jagger and Lionel Pearson. Its portland stone machine guns and hooded-eyed, beaten-down figures do not come across as misplaced or kitsch, but profoundly disturbing. Of all the many memorials in the area it is the hardest to walk past unthinkingly – as soon as you notice it, you're shaken up, captivated by its intensity and sense of horror. It would be pointless to deface it to remind passersby of the evils of war, as they are conveyed already.